In 1937 Albright and co-workers suggested that rickets or osteomalacia might result from deficient action of vitamin D. With recognition that vitamin D is the precursor to 1,25(OH)2D, a steriodal hormone, it has become possible to identify patients with defective activation of vitamin D and others with defective target action of 1,25(OH)2D. We have shown that patients with hereditary resistance to 1,25(OH)2 exhibit a spectrum of defects in interaction of 1,25(OH)2D with cultured skin fibroblasts. Skin fibroblasts provide a useful model for binding and action of 1,25(OH)2D in its classical target tissues (duodenal mucosa, etc.).